Laser along-body tracker comprising laser beam dithering

ABSTRACT

A non-imaging laser-based tracking system for tracking the position of a targeted moving object. The tracking system includes two lasers: a reference laser and a slave laser. Each laser is a weapon, and when locked on a target, single laser effectiveness may be doubled without a thermal blooming performance loss associated with a single laser operating at twice the power. The slave laser beam is dithered relative to the reference laser beam in a direction along the longitudinal axis of the target. The system includes an optical receiver for repetitively scanning the irradiance profile reflected by the target. Since the slave laser beam is dithered relative to the reference laser beam, both laser beams will jitter and drift together providing a gain factor of two in average irradiance on the moving target.

This application is a related to copending patent application, Ser. No.08/631,645 filed on Apr. 2, 1996, now U.S. Pat. No. 5,780,838, for aLASER CROSS BODY TRACKING SYSTEM AND METHOD (LACROSST), by Peter M.Livingston and Alvin D. Schnurr.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to systems for tracking a movingobject and, more particularly, to a non-imaging laser-based system fortracking a moving object that employs two lasers; a reference and slavelaser. Each laser is a weapon, and when locked together on a target,single laser effectiveness may be doubled without a thermal bloomingperformance loss associated with a single laser operating at twice thepower. The slave laser is dithered relative to the reference laser in adirection along a longitudinal axis of the target. The system includesan optical receiver for repetitively scanning the irradiance profilereflected by the target.

2. Description of the Prior Art

Various types of systems are known for tracking moving objects, such asrockets and missiles. Such systems can be categorized as either imagingor non-imaging. Imaging types systems normally utilize an imagingdevice, such as an electronic camera, for detecting and tracking theposition of a targeted moving object. While such imaging systems areeffective in tracking targeted moving objects, such imaging systems areknown to have limitations when used in combination with high power laserbeam weaponry. For example, in such systems, the high power laser beamis known to interfere with the imaging system potentially causing lossof the track of the targeted moving object. Although various systems areknown compensate for such an interference problem, such systems do noteffectively eliminate the interference.

As such non-imaging laser type tracking systems are known. An example ofsuch a system is disclosed in copending U.S. patent application no.08/631,645 filed on Apr. 2, 1996, entitled LASER CROSS BODY TRACKER(LACROSST), assigned to the same assignee as the assignee in the presentinvention, now U.S. Pat. No. 5,780,838. The system disclosed in the '645patent application includes a laser generator for generating a singlebeam of laser energy and a beam steerer for steering the beam of laserenergy to track a targeted moving object. The beam steerer steers thebeam of laser energy in a oscillatory fashion in two orthogonaldirections at a first dither frequency and a second dither frequency,respectively. The system also includes a telescope for receivingreflected laser energy from the targeted object and detecting the amountof reflected energy received. The detected energy is filtered to formfirst and second dither frequencies for each channel. The filteredsignals are synchronously detected by multiplying each channel by asinusoidal function derived from the laser mirror generator for thatchannel. A bias signal is generated from the received reflectedsynchronously detected power proportional to the beam centroiddisplacement from the target midline which allows the beam steerer tosteer the laser beam to center it on the target, thereby tracking thetargeted object.

Unfortunately, non-imaging laser based tracking systems are subject towhat is known as thermal blooming. Thermal blooming results in a changein the refractive index of the beam path as a result of heating the beampath temperature by the laser. Change of the refractive index creates alens effect that causes the radiation to spread relative its originaldirection. As such, thermal blooming increases the diameter of the laserbeam as it moves away from the laser source. A detailed explanation ofthe thermal blooming is disclosed in U.S. Pat. No. 5,198,607, herebyincorporated by reference.

The problem of thermal blooming also reduces the effectiveness of highpower laser weaponry. In order to overcome the thermal blooming problemfor high power laser weaponry, the '607 patent discloses the use of twoindependent lasers separated by a sufficient distance to preventinterference therebetween, focused onto a single moving object, such asa missile. The '607 patent discloses the use of a known imaging typesystem for tracking the location of the targeted moving object.

SUMMARY

It is an object of the present invention to solve various problems ofthe prior art.

It is yet another object of the present invention to provide anon-imaging tracking system for tracking the position of a targetedmoving object.

It is yet another object of the present invention to provide anon-imaging type tracking system which reduces the effects of thermalblooming.

Briefly, the present invention relates to a non-imaging laser basedtracking system for tracking the position of a targeted moving object.The tracking system includes two lasers: a reference laser and a slavelaser. Each laser is a weapon, and when locked together on a target,single laser effectiveness may be doubled without a thermal bloomingperformance loss associated with a single laser operating at twice thepower. The slave laser beam is dithered relative to the reference laserbeam in a direction along the longitudinal axis of the target. Thesystem includes an optical receiver for repetitively scanning theirradiance profile reflected by the target. Since the slave laser beamis dithered relative to the reference laser beam, both laser beams willjitter and drift together providing a gain factor of two in averageirradiance on the moving target.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantages of the present invention willbecome readily apparent upon consideration of the following detaildescription and attached drawing, wherein:

FIG. 1 is a diagram illustrating the reflected radiation from auniformly reflecting body illustrating two overlapping laser spots.

FIG. 2A is a graphical representation of the irradiance profile as afunction of the distance along the reflected body for twonon-overlapping laser spots.

FIG. 2B is a graphical illustration of the irradiance profile asfunction of the distance along the reflecting body where the twooverlapping laser spots are overlapping as illustrated in FIG. 1.

FIG. 3 is a diagram of a single laser beam overlapping a highlyreflecting feature on a target.

FIG. 4 is an overall block diagram of the non-imaging laser track systemin accordance with the present invention.

FIG. 5 is a block diagram of an optical receiver and image derotator inaccordance with the present invention.

FIG. 6 is a detailed block diagram of the tracking system in accordancewith the present invention.

FIG. 7A is a graphical illustration of the received scan signal for fourcomplete scan cycles of the tracking system in accordance with thepresent invention.

FIG. 7B differentiated version of the signal illustrated in FIG. 7A.

FIG. 8 is a graphical illustration of the power spectral density of thesynchronously detected scan signal at twice the frame rate for thetracking system in accordance with the present invention.

FIG. 9 is a graphical illustration of the detected dither modulationsignal for the tracking system in accordance with the present invention.

FIG. 10 is a graphical illustration of an error characteristic signalfollowing the second detection of the 5 Hz dither modulation signal.

DETAILED DESCRIPTION

The present invention relates to a non-imaging laser based trackingsystem for tracking a targeted moving object. In order to minimize theeffects of thermal blooming, the tracking system in accordance with thepresent invention employs two laser beams to detect the location of atargeted moving object. One laser beam is configured as a reference beamand is directed toward the targeted moving object. The other laser beamis configured as a slave and is dithered (oscillated with a smallamplitude) relative to the reference laser beam on the target surface ina direction generally parallel to a longitudinal axis of the target. Asshown in FIG. 1, such a system will result in overlapping laser spots onthe moving target. Since the reference and slave laser beams are lockedtogether, both beams will jitter and drift together producing a gain inthe average irradiation profile of two as illustrated in FIG. 2B. Moreparticularly the irradiance for separated laser spots on a target isillustrated in FIG. 2A. By causing the laser spots on the target tooverlap as illustrated in FIG. 1, the irradiance gain illustrated inFIG. 2B is about twice the normal gain as illustrated in FIG. 2Aassuming equidistant laser paths and equal laser powers.

FIGS. 1 and 2 illustrate the irradiation for a uniformally reflectingbody. FIG. 3 illustrates the use of the invention with a non-uniformallyreflection body. In particular, referring to FIG. 3 a single laser beamcan be locked on to shiny or dull feature of a target. Even though thetotal number of photons scattered from the target is constant, theinvention will process the information as to lock the single beam ontothe distinguishing feature as will be discussed in more detail below.

A system for directing a reference laser beam at the target of interestis described in U.S. patent application Ser. No. 08/631,645, filed onApr. 2, 1996, now U.S. Pat. No. 5,780,838, assigned to the same assigneeas the assignee of the present invention and hereby incorporated byreference. The present invention as described below is related tolocking a second laser beam (slave beam) relative to the reference laserbeam on the target surface in a direction generally parallel to thelongitudinal axis of the target.

Referring to FIG. 4, a targeted moving object such a missile or rocket20 is illustrated defining a longitudinal axis 22. A first laser 24 isused to direct a first laser beam 26 toward the target 20. For purposesof illustration herein, the first laser 24 is designated as thereference laser. In accordance with an important aspect of theinvention, a second laser 28 is used to direct to a second laser beam 30toward the target. The second laser beam 30 is dithered (i.e. oscillatedwith a relatively small amplitude) in a direction as indicated by thearrows 32 that is generally parallel to the longitudinal axis 22 of thetarget.

The tracking system forms a closed loop that forces the laser spots fromthe laser beams 26 and 30 to overlap as illustrated in FIG. 1 for alldisturbance frequencies falling within the loop bandwidth. The closedloop system in accordance with the present invention is generallyidentified with the reference numeral 34 and includes an opticalreceiver 36, a signal processor 38, a dither generator 40 and a dithermirror 42. Unlike the single laser tracking system disclosed in thecopending application 08/631,645 for the LASER CROSS-BODY TRACKER(LACROSST), now U.S. Pat. No. 5,780,838, the tracking system 34 does notdepend on the time variation of the total number of scattered photonsbut rather information derived competitively scanning the irradianceprofile created the two laser beams 26 and 30 directed toward the target20. The signal processor 38, as will be discussed below in more detailis used to process the scan data from the optical receiver 36 in orderto lock the beam 30 from the slave laser 28 to the beam 26, generated bythe reference laser 24.

The optical receiver 36 is illustrated in detail in FIG. 5. The opticalreceiver 36 is used to detect the irradiance profiles for example asillustrated in FIG. 2B, reflected from the target 20 by imaging a target20 with its laser spots at the laser site using a rocking mirror scannerassembly as discussed below. The optical receiver 36 includes a primaryafocal telescope 44 for receiving scattered laser energy from arelatively wide field-of-view, for example, a field of regard of severalhundred microradians. The scattered laser energy from the target 20 isdirected by the afocal telescope 44 to an image derotator 46. The imagederotator 46 is used to cause a rocking mirror scanner assembly 48 toscan the scattered laser energy in a direction generally parallel to thelongitudinal axis 22 of the target 20. More particularly, the opticalreceiver 36 includes a single detector 50 having a field stop slit 52which defines the system instantaneous field-of-view. The rocking mirrorscanner assembly 48 causes the image of the overlapping spots on thetarget 20 to be swept over the slit 52. The image derotator 46 thus isused to force the dither direction to be generally perpendicular to thefield stop slit 32. The field lens 54 is used to direct the image on thesingle detector 50. In order to improve the signal to noise ratio, azoom lens assembly 56 may be used in conjunction with a system radarrepresented by the dashed line 58, to fill the field stop slit 52 to thegreatest possible extent. In order to ensure that the spot image on thedetector 50 remains centered, bias signals from the signal processingsystem 38 may be used. The bias signals may be generated by the signalprocessing system 38 resulting from the action of the tracking systemservoloop 34. The action of the servoloop causes the optical signal tobe centered on the detector and eliminates signal drift perpendicular tothe direction of scan.

The optical receiver 36 may be mounted to the coarse gimbals of a laserbeam pointer system. The laser beam pointer system is described indetail in U.S. patent application Ser. No. 08/631,645, filed on Apr. 2,1996, now U.S. Pat. No. 5,780,838, assigned to the same assignee as thepresent invention and hereby incorporated by reference.

The signal processing system 38 is illustrated in FIG. 6. A scan mirrorgenerator 53 (FIG. 6) as well as the scan mirror drive 55 form part ofthe signal processor 38 discussed below which drives the rocking mirrorscanning assembly 48. The scan mirror generator 53 causes the scanmirror to move an approximately constant angular rate scanning the imageback and forth over the fixed slit 52. The detector 50 thus records avoltage proportional to the irradiance filling the slit 52. The slitwidth and scan extent together with the zoom lens assembly 56 cause theimage to be moved completely out of the field of view and returned. Thedetected voltage from the detector 50 thus represent a running integralof the irradiance distribution.

The output from the detector 50 is applied to a detector preamplifier52. The signal processor 38 is used to develop a scan signal for eachcomplete scan cycle as illustrated in FIG. 7A. The signal processor 38includes a pair of differential amplifiers 54 and 57 as well as a pairof commutating switches s₁ and s₂. As mentioned above, the scan mirrormoves at a constant angular rate scanning the image back and forth overthe fixed slit 52. In a forward scanning direction, the commutatingswitches s₁ and s₂ cause the signal to be applied to the differentialamplifier 54. In particular, when the commutating switch s₁ is closed, asignal from the detector/preamp 52 is applied to a noninverting input ofa differential amplifier 54 and compared with the output of thedifferential amplifier 57 which is zeroed by a capacitor c₂, connectedbetween the output of the differential amplifier 57 and ground. Adischarge resistor R₂ is used to discharge the capacitor such thatoutput of the differential amplifier 57 is zero when the commutatingswitch s₁ is closed. Thus when the commutating switch s1 is closed, thesignal from the detector/preamplifier 52 will be positive. Thecommutating switch s₂ is used to connect the output of the differentialamplifier 54 to a differentiator 59 in the forward direction of the scancycle to produce the portion of the signal illustrated in FIG. 7A with apositive slope.

In the return direction of the scan cycle, the commutating switch s₁causes the signal from the detector/preamplifier to be connected to aninverting input of the differential amplifier 57. As mentioned above,the capacitor c₂ in combination with the discharge resistor R₂ causesthe output of this amplifier to be zero prior to the rocking mirrorscanning assembly 48 moving in a return direction a noninverting inputof the differential amplifier 57 is connected to the output of thedifferential amplifier 54. In a return direction, a capacitor c₁ andparallel connected discharge resistor R₁ force the output of thedifferential amplifier to be zero in the return direction. Thus, in areturn direction, the detector preamp signal 52 is merely inverted asillustrated by the negative slope of the scan signal for the last halfcycle as illustrated in FIG. 7.

The scan mirror generator 53 is connected between the commutatingswitches s₁ and s₂ to control their operation. More particularly, thescan mirror generator 53 is coupled to a scan mirror drive which, asdiscussed above causes the rocking mirror scanner assembly 48 to scanthe image back and forth over the fixed slit 52. The scanning mirrorgenerator 53 also controls the operation of the commutating switches s₁and s₂ as discussed above. The waveform illustrated in FIG. 7A, shownfor four complete scan cycles is thus produced at the output of thecommutating switch s₂.

A scan signal, as illustrated in FIG. 7A, is applied to thedifferentiator 59. The differentiator 59 differentiates the scan signalto produce a differentiated scan signal as illustrated in FIG. 7B.Except for the line reversal of the odd half cycles, the differentiatedscan signal illustrated in FIG. 7B is similar to the irradiance profilefor partially overlapping laser spots on a target illustrated in FIG. 2.Since the differentiation eliminates the DC component, operation of thesystem does not depend on accurate DC restoration.

After the scan signal is differentiated, the dither modulation portionof the signal, which shows up as an FM component, is recovered. Inparticular the differentiated signal illustrated in FIG. 7B issynchronously detected by multiplying it with a cosine signal from thescan mirror generator 53. More particularly, referring to FIG. 6, theoutput of the scan of mirror generator 53 is applied to a frequencydoubler 58. The output of the frequency doubler 58 is essentially a sinewave at twice the sweep frequency of the scan mirror generator 53. Theoutput of the frequency doubler 58 is applied to a 90° phase shiftingdevice 60 which, generates a cosine signal at twice the sweep rate. Theoutput of the phase shifter device is applied to a multiplier 62.

The power spectral density of the synchronously detected scan signal attwice the frame rate is illustrated in FIG. 8 at a 0.6 beam widthseparation. As shown in FIG. 8, the frame rate is 40 frames or 80 scansper second. Thus, for a scan rate of 80 Hz synchronous detection onlydetects the sidebands 40 Hz above and below the 80 Hz above and belowthe 80 Hz signal within the synchronously detected differentiated scansignal at the dither frequency of 5 Hz.

The synchronously detected differentiated scan signal is applied to alow pass filter 64 as illustrated in FIG. 6. The output of the low passfilter 64 is applied to another multiplier 66 used for synchronousdetection at the dither frequency to recover the signed envelope of thedither signal. A signal from a dither mirror scan generator 68 isapplied to the multiplier 66. The dither mirror scan generator 68 isused to drive a dither mirror drive 70, which, in turn, drives thedither mirror and, in turn, the slave beam 30 at the dither frequency.

FIG. 9 illustrates five synchronously detected 5 modulation signalsshown for 7 values of spot separation 0.0 through 0.6 at 0.1 increments.As shown, the dither modulation envelopes illustrated in FIG. 9 aresigned. Thus, referring to FIG. 1, if the dithered spot is to the leftof the reference spot and moves toward it as the high power beam faststeering mirror 42 (FIG. 4), moves from left to right, the envelope signis positive. However, if the dither spot is to the right of thereference spot, the same motion of the high power fast steering mirror42 causes the sign to be negative.

The sign of the dither modulation signal then may be used as an errorsignal to lock the reference beam 30 from the slave laser 28 relative tothe reference beam 26 from the reference laser 24. As shown in FIG. 10,the error signal changes magnitude with the mean spot displacementmeasured in beam radii. FIG. 10 is an example of the detected 5 Hzdither modulation signal recovered for seven values of mean spotdisplacement. As shown in FIG. 10, the system will cause the beam 30 towalk onto the reference beam 26 for least plus or minus 0.6 beam-radiusseparations. As shown in FIG. 6, this detected signal is applied to anintegrator 70 which forms a closed loop with the dither mirror scangenerator 68 drive the dither mirror with a signal proportional to theintegral of the detected envelope.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A non-imaging type tracking system for tracking amoving object having a longitudinal axis, the tracking systemcomprising:a system for generating a first laser beam and tracking saidpredetermined object with said first laser beam; a second laser forgenerating a second laser beam; and means for locking said second laserbeam relative to said first laser beam to cause overlapping laser spotson said moving object, said locking means including means for ditheringsaid second laser beam relative to said first laser beam at apredetermined dither frequency, wherein said dithering means includesmeans for causing said second laser beam to move in a directiongenerally parallel to said longitudinal axis of said moving object. 2.The non-imaging type tracking system as recited in claim 1, wherein saidlocking means includes means for detecting scattered laser energy fromsaid moving object.
 3. The non-imaging type tracking system as recitedin claim 2, wherein said detecting means includes a predeterminedtelescope.
 4. The non-imaging type tracking system as recited in claim3, wherein said predetermined telescope is a primary afocal telescope.5. The non-imaging type tracking system as recited in claim 3, whereinsaid detecting means includes a detector and means for imaging saidoverlapping laser spots on said detector.
 6. The non-imaging lasersystem as recited in claim 5, wherein said imaging means includes amirror scanner for scanning said scattered laser energy at apredetermined scan rate defining scan signals.
 7. The non-imaging typetracking system as recited in claim 6, further including means forsynchronously detecting said scan signals.
 8. A method for tracking amoving object comprising the steps of:(a) providing a first laser beamwhich tracks a predetermined moving object; (b) providing a second laserbeam that is dithered with a relatively small amplitude periodic motionrelative to the first laser beam on the object surface in a directiongenerally parallel to a longitudinal axis of the object; and (c)repetitively scanning the irradiance distribution created by the twolaser beams directed to a single object to form a detected signal.